Standard global navigation satellite systems (GNSS) such as GPS, GLONASS, or the future European Galileo system provide suitable positioning information enabling worldwide a determination of position, velocity and time with a good accuracy, depending on the available GNSS information and services (civil signals service, military or public signals service, etc.). The GNSS signals enable distance computation between receiver and transmitter within the standard Radio Navigation Satellite Service (RNSS) that allows for three-dimensional navigation if at least four ranges to different satellites are available, and if the locations of these satellites are a priori known.
Due to various effects between signal source and the receiver, the range computation might suffer from the following effects: inaccurate satellite location modeling, signal delays due to atmospheric effects, local reception effects like multipath, etc., especially over the large distance between GNSS satellites in orbit and user receivers on the ground, the geometrical range is only estimated with an accuracy that is degraded by the mentioned error contributions. Depending on the available signals from the GNSS services and on the local environment of the user, the range can be estimated within an accuracy of 1 to 3 meters, but due to the above described issues also errors higher than 20 to 30 meters are likely to occur. Such range information accuracy typically allows for a three-dimensional localization accuracy in the order of 10 m in absolute earth inertial coordinates.
Such a positioning performance sufficiently supports many applications or services, like car navigation or even airplane en-route navigation, but does not allow for high-accuracy missions that require centimeter-level accuracy, like aircraft or helicopter touch down situations or critical docking maneuvers, etc.
There are ways to improve a GNSS regionally with satellite based augmentation systems (SBAS) like WAAS or EGNOS or locally with local area augmentation systems (LAAS) through reference stations and correction data or Pseudolite consideration. These systems improve the accuracy of the satellite range information and thus the user's positioning performance, but these concepts are either not accurate enough (SBAS), or very complex and expensive to install and to operate (LAAS). Furthermore these complex regional or local GNSS signal based systems only improve the absolute localization of the user, and cannot or only through additional and again very complex system be used in environments respectively for missions where the target (e.g. a helicopter landing platform) is moving and/or is even changing its attitude.
Exemplary embodiments of the present invention provide locally restricted and suitable distance information from locally distributed and temporarily relative to each other fixed (at least fix for mission duration) transmitters allowing for highly accurate relative localization.